Signal control



.AP 7, 1942- y LE'RoY .cj PASLAY srGNALfcoNTRoL Filed May '9, 1939 r 2 sheets-sheet 1 ATTORNEYS LE ROY PAsLAY 2,279,128

April 7', 1942.

SIGNAL CONTROL Filed May s, 1959 sheets-sheet 2 Patented Apr. 7, 1942 2,279,128 l SIGNAL CONTROL Le Roy C. Paslay, Dallas, Tex., assigner to National Geophysical Company, Dallas, Tex., a.

corporation of Delaware Application May 9, 1939, Serial No. 272,563

' (ci. 17e-1v1) 7 Claims.

This invention relates to alternating current transmission systems, and more especially to the control of current in such systems wherein thermionic tubes are employed, as, for example, in signalling systems. i

The invention is especially advantageous in connection with amplifier systems .designed for use. in geophysical exploration work, and coinprises an' `iinpiovenient. on the invention described in my application for U. S. Letters `Patent .Serial :Number 159,994., filed August 19, 1937.

The control of alternating current transmission, especially of signals whether of low frequency or high frequency, has been the subject of considerable investigation by those skilled in the art. Such investigations have included both manual and automatic controls, the latter usual- 1y :having been referred to as automatic gain controls or automatic volume controls. Practically all `successful automatic gain control `systems have been based on .the principle of amplifying the signal in a grid-controlled thermionic tube, rectifying part of the amplified signal and applying the rectified signal, as a biasing potential, to the control grid of the signal amplifier. A large number of'inodications and variations of that funda-mental system have been devised. However, it has been recognized that the control of gain in that :manner inevitably produces a certain amount .ofldistortion and other attendantdisadvantages of a nature depending upon the circumstances. In certain control systems prior to vthe present invention some of these disadvantages resulted. vfrom the reaction of the control'current upon the current being controlled when the apparatus involved became unbalanced, and Yinotiier systems a disadvantage resulted from the fact that if the gain control potential were to be .applied to a reasonably small .number of .amplifier tubes the range of control could not beularge 4 witliout introducing distortion of a natu-re depending upon the amplication characteristics of the thermionic tubes employed. These and many other .disadvantages in the prior gain control systems are overcome by the system of Ythe present invention.

The control, and especially the automatic control, of gain is di-fhcult in respect to low frequencies. This is particularly true in lthe event tlfiat the low frequencies .comprise unmodulated signals. For instance, in geophysical exploration Work A.ccinprising the seismometric recording of earth vibrations resulting from the detonation of an explosive, the `frequencies involved may cover a range of between about 30 and 200 cycles per second while the amplitude of the vibrations may vary as much as 20,000 to 1 in 5 seconds, or so. Graphical records of such variations are diflicuit or impossible to make and to read when the 'amplitude of the vibrations has decreased to a low value, although frequently these very vibrations' have important significance to a geophysicist. Consequently it is important,`rst, that the signals be amplied more as they become weaker and, second, that a minimum of distortion be introduced in doing so. The present invention meets these requirements with considerable success, these and other of its valuable features being applicable both to manual and automatic gain control systems. For a more detailed discussion of geophysical 'work'and apparatus, and the manner in which automatic gain control would be employedtherein, reference is made to my mentioned patent'application.

The preferred form of the present invention contemplates the automatic control of alternating current or signals in such manner that the control system itself introduces no appreciable effect in the signal transmission channel except the desired control of transmission or gain. In accordance with the invention, the alternating current or signal is controlled by a gain control or .an attenuatingelement responsive to 'another orV control current `having characteristics separably dierent from those of the current or signal being controlled.

A .better understanding ofthe invention will be had from a `consideration of the following specification, and from the drawings in which:

Fig. 1 is a block diagram of an embodiment of this invention wherein the Vprincipal elements are labeled;

Fig. 2 is a circuit vdiagram of a system more generally represented in Fig. l; and

Fig. 3 is an equivalent diagram ofthe gain control element represented in Figs. 1 and2.

Referring first to Fig. l, there are shown eight blocks, those numbered l, 2, 3 and 8 representing elements. in an alternating-current transmissionor signal channel, and those numbered 4 5, ,6 and l representing elements in a control channel.

.In the embodiment shown herein by way of a preferred example, it is assumed that the signal or other alternating current is to be automatically controlled. Accordingly, the signal is first amplified in amplifier I, and is then passed through the gain control element 2 where the current to be controlled (hereinafter for convenience referred to as the signal) is' attenuated in the desired manner. This gain control element preferably is of a type having the characteristic that its control effect varies with variation of current through' it. The controlled signal then passes through a second amplifier 3 and thence into `an appropriate utilization device 8, which, in the assumed case of geophysical exploration apparatus, would usually be a graphical recording device.

Still referring t Fig. 1, the output of the sec-V increased as the `effective resistance decreases.

ond am lifier 3 is also coupled to a rectifier 4,v p attenuating elements II and I2 connected in the rectified current from which actuates a direct-current amplifier 5. Also included in the control channel is a local oscillator., 1 which supplies alternating current to theinput of an amplifier 6 at a frequency of an order of magnitude different from that of the signals, this.

Before describing the components of this portion (2) of the system in detail, reference is made to Fig. 3 which represents in simplified form the equivalent of the components represented between terminals a, b, and c, d of Fig. 1. Resistance elements having non-linear characteristics are represented in Fig. 3 by variable resistors II, I2. Fromthis it can readily be seen that signals transmitted through the channel between a, b and c, d will be attenuated in accordance with the effective resistance of the shunt in the signal channel. When the attenuating elements have a lower effective resistancethan the series resistors R6 and R1 the attenuation will be `much greater than when the 5- attenuating elements have a higher effective refrequency preferably being considerably higher than thai-l of the signals. For reasons later to be explained in connection with Fig. 2, control-current'amplier 6 is arranged not only to amplify the output of the oscillator, but also to double the frequency thereof. The output circuit of amplifier 6 is connected so that the output current therefrom flows through gain control element 2, thus controlling the attenuation effect introduced by that element in the signal channel. Direct-current amplifierr 5 is so coupled to control-current amplifier that the output current of amplier 6 is caused to vary in accordance with the variation of the direct potential developed in the output circuit of amplifier 5. Thus the gain control effected by element 2 may be said toy bea function of the variation of the output of 'rectier `4.

The circuit vdiagram of a system which follows the generalarrangernent of Fig. 1 is shown inFig. 2 wherein sections of the components are enclosedin dotted lines forming blocks identified by reference characters which correspond to those of Fig. 1. Accordingly, a rst amplifier 1 is shown coupledY to the source 'of signals by an input coupling transformer I8 designed to accommodate the frequency range of the signals .(here assumed tobe approximately to 200 cycles). The amplifier comprises two stages ofthe resistance-coupled type. Thermionic amplifyingtubes 9 and I0 are here shown for simplicity as being separate triodes, although in` practice they might comprise a duplex triode, such as the type 6C8. Biasing resistors RI and R4 together with their by-pass Condensers CIk and C3, respectively, are of the values and types usually employed with tubes of thisY character.

Suitable values for the anode and grid resistors R2 and R3, respectively, and coupling condenser C2 are given in a table set forth Hereinafter, as

are the electrical values of other of the components. K

Gain control element 2 is coupled to the output of amplifier I through resistor 5 and condenser C4. This gain control, or attenuating, element may include a pair of resistance or attenuating elements II, I2 having non-linear characteristics. The effective resistance of such a non-linear element varies in accordance with the* current vflowing therethrough, and in this instance the effective resistance decreases with increase of current therethrough, although Such relation not usually a direct proportion. In orderv to effect the desired control of attenuation the 4`elements shown are connected in shunt to thesignal channel so that the `attenuationis condenser C6 and resistors RIS and RI1'comsistance than the resistors R6 and R1. The present invention provides means for controlling the effective resistancesA of the attenuating elements without undesirably affecting or otherwise changing the characteristics of the signals.

Referring again to Fig. 2, the variable resistance elements II and I2 may comprise doublediode thermionic tubes such, for example, as the type 7A6, although any suitable non-linear impedance elements, capacitive, inductive or resistive, may be substituted. For example, if rectiiiers of the copper oxide type be substituted they would also be connected as shown in Fig. 2, so that the alternating current will flow through theV device in both directions. On the other hand, if a resistor of ay non-linear type such as that known as Thyrite be employed, only two is, in accordance with this invention, varied and controlled by passing through such elements a current having characteristics sufficiently different from those of the signals to make practicable effectively complete separation of the two currents,thus avoiding any distortion or other undesirable effect .on the signals bythe control current. To this end, an alternating control current of, say, 400,000 cycles is supplied through leads 32 to a circuit comprising inductances I9 and 2I and variable condenser 3| which is tunable to 400,000 cycles. Inductances I 9 and 2I comprise the primaries of transformers of which thesecondaries are 20 and 22, respectively.V Consequently, the alternating control current at that frequency is thus caused to flow in secondaries 20 and 22,' respectively, and hence in common through the circuit 20, C5 and I I and the circuit 22, C1, and I2. values offeringvery high impedance to the signal frequency but low impedance to the controlcurrent frequency. The inductance of secondaries 20'and 22 is so s'mall as to offer essentially v no impedance at the signal frequency. Battery 28 which may be a part of battery 21 and may, therefore, be tapped from a' suitable voltage divider of'- a direct-current power supply system. provides a biasing potential to each of tubes II andIZ in order to bias those tubes just to the point of essentially infinite resistance. By-pass Condensers C5 and C1V are of,

plete this biasing system. An equivalent bias could be obtained by omitting components 28, RIB, RI'I and C6 and substituting separate biasing batteries correctly Iconnected as to polarity between either the anode or the cathode of each of the double diodes and the common return lead shown as connecting terminals b and d. After the correct adjustment of such a biasing potential is made, if any current whatever flows through the diodes the internal resistance thereof is effectively decreased. Such a decrease isof course, effective in regard to the signal current, which being of so low a value across the nonlinear element does not cause a suiiicient change in resistance to produce distortionv of the signal.

After .the signal passes through the gain control element 2, it is amplified in the second amplifier 3 which may be substantially identical with the first `amplifier I. The output circuit of the second amplifier includes primary winding 24 of an output transformer 23 which should be designed for the frequency range of the desired signals. This transformer may include two secondary windings, namely, secondary 25 which may be connected to a vsignal utilization device 8, and another secondary 25 which may be connected to .a rectier 4. In this instance a full-wave rectifying device I isernployed, and hence secondary 26 includes a center tap connection. Any suitable `full-wave or half-wave rectifier may be used for this purpose. Thus, a predetermined partof the signal energy appearing in transformer 23 is caused to fiow through rectifier 4 and be rectified into direct or pulsating unidirectional current. Any undesired pulsations in the direct-current output of rectifier 4 may be filtered out, if desired, by the use of filter elements well known in the art.v

The output of rectifier 4 is, in this instance, connected to the input of a direct-current amplifier 5 comprising a thermionic tube I6 and associated elements. These elements include voltage divider resistors RI3 and RI4, the terminals of which are connected across the anode-potential battery or voltage source 21 which supplies the anode potentials for all of the thermionic tubes. Reactor 30 and condenser CI2 together comprise a filter which will remove alternating current components from the amplified direct current, the inductance of reactor 30 and the capacitance of condenser CI2 being both high. The rate at which condenser CI2 charges is determined largely by the anode resistance of tube I6 plus the resistance of RI3; while the discharge rate of CI2 depends largely on the load drawn by the therrnionic tube I1, i. e., on the effective resistance of the anode circuit of tube I1. In the particular system herein illustrated the constants controlling the rate of charge and discharge of condenser CI2 have been chosen to cause rapid depression to strong signals and slow recovery to weak signals. In selecting the value of condenser CIZ the fact should also be borne in mind that this condenser determines in part the rate which the automatic gain control action incurs upon each change in the magnitude of the signal, The battery 29, or other appropriate source of potential, furnishes a bias beyond cutoff for tube I6 so that an initial potential of about 1.5 volts output from rectifier 4 is required before tube I6 starts to function. rIhis furnishes the required delay in order that the system produce the comparatively flat automatic gain control characteristic which is vdesirable in this particular embodiment.

` put circuit of tube I1.

In the event thata `manual control I.be desired, instead of the automatic control herein illustrated,.it is necessary only .to substitute for elements 4 and `5 an equi-valent source of manually controlled ydirect-current potential across the condenser CI2.

A- local oscillator represented :byblock J comprises any convenient source of alternating current and hence has ,not been shown in detail. Such independent source :of alternating current, which in this case may be assumed to have la frequency of 200,000 cycles, is coupled by means of condenser CI3 and resistor RIS to `controlcurrent amplifier 6. This control-current amplifier includes thermionic tube I-T, which for convenience may comprise one section of -a duplex triode tube such as type 608. VThe other section of such tube may comprise tube I6 which for simplicity has been drawn as a separate tube.

It will be noted that the output circuit of amplifiertube I'I includes the tuned circuit I9, 2.I 3| and condenser CIZ. The only source of anode potential for tube I'I is the direct-current potential appearing across condenser CIZ, butv this condenser being .of high capacitance .offers negligible impedance to alternating current of the frequency .appearing in the mentioned output circuit. This direct-current potential even when it fluctuates in accordance with the output of rec-V tier 4, never reaches a highk Value, so that actually, the anode potential supplied to tube I1 is always greatly lower than would be `specified for normal operation of a tube of the type employed in this control-current amplifier. For-this reason a comparatively small change in value of direct-current potential across condenser CI2 will effect a rather large change in the magnitude of control current supplied to the coupling circuit which includes condenser 3|.

By reason of the circuit constants, grid bias and anode potential employed in connection with tube Il, a rather large lcurrent component at double the input frequency Aappears in the out- Thus, in practice, the tuned circuit I9, '2 I, 3'I, maybe adfrusted to 400,- 000 cycles, which -ink this instance will be a much higher frequency than the signals. Consequently, because of lthe values of the circuit components this comparatively high frequency of 400,- 000 cycles will normally be confined to the gaincontrol element 2. Sometimes, however, it `may be desirable to connect in each or either of ampliers I and 3 a band-pass filter tuned to pass only the signal frequency or the range of signal frequencies.

With the system above described it is Apossible to achieve a very complete control of the high-frequency control current without neutralizing the grid-anode capacitance of tube I1, for the following reasons: As shown, the resonant circuit I-9, 2I, 3l istuned to approximately 400,- 000 cycles, so that substantially no potential at 200,000 cycles is developed in the output circuit of tube I7, whereas a potential at 400,000 cycles is developed in that circuit. This 400,000 cycle current, being the result of the frequencydoubling function of tube I1, exists in the anode circuit only by reason of the tubes thermionic action and therefore is subject to control Aby variation of the anode potential independent of any interelectrode capacity coupling. Under different circumstances equivalent results may be had by substituting a neutralized triode or a pen. tode having a shielded grid, and employing such a tube as a straight amplifier and not as a doubier-'as well.V In that event the oscillator should be adjusted to supply control current at the same frequency as that to which the coupling circuit I9, 2|, 3| is totbe tuned. If, on the other hand, the'oscillator frequency be dictated by other considerations, that frequency might be set as required, and the control-current amplifie'r'operated as a frequency Adoubler even though it be `neutralized or comprise a shielded grid pentode. When the capacity coupling effect is minimized the direct-current control potential may be impressed on the control grid instead of inv` the lanode circuit, although this is usually not to be preferred.

wTo complete the foregoing description of the specific apparatus represented in Fig. 2, but without implying any limitation as to the scope of theinvention, the following electrical values of circuit elements are suggested:

RIG- 4,000 ohms --320 henries RI 1-2,000 ohms The operation of the transmission system above described is, briefly, as follows: With the oscillator 1 generating a frequency of say 200,000 cycles, a signal preferably of fixed amplitude is impressed on input transformer |8. Variable condenser 3| is then adjusted slowly until the amplitude of the signal appearing at the output 8 of the transmission channel is at a minimum. This will usually occur when the adjustment of condenser 3| has tuned the coupling circuit I9, 2|, 3| to a frequency slightly different from that of 'the current appearing in the leads 32. The apparatus is then ready for use. Assuming that the circuit components are of the values above stated and that no signal is present in the signal channel, an impressedsignal, in the first instant, will be amplified in amplifier and will pass through the gain control element 2 without 'appreciable attenuation because the resistance of the attenuating elements and I2 will then be very high. The signal will next be further am-r plified by amplifier 3 and will then pass out through secondary 25 of output transformer 23 to operate a recorder or other device responsive to the signals'. Part of the amplied signal will be rectified by rectifier 4 and will be amplified by the direct-current amplifier 5. In the system illustrated the potential supplied to the anode circuit of tube |1 will be sufficient to allow control current to iiow in the leads 32 when the rectifier output reaches'about 1.5 volts. As the voltage 'in the output circuit of rectier 4 increases, the control current in the leads 32 increases untilit reaches a maximum, thus resulting in maximum signal attenuation when the output voltage from the rectifier reaches about 1.9 volts. Such a variation of about 0.4 volt from the rectifier represents a variation in signal strength of roughly'20,000 to 1, which means that a signal-strength variationof that order of magnitudek may be automatically controlled with substantially no signal distortion. Conversely, as

- thus decreasingthe attenuation of the signals in proportion.

i I claim: i A 1. Y,In a'signallingsystem, a signal channel including a first signal amplifier, a signal gain con-` trolelement, a second signal amplifier and a. signalutilization device coupled in the order named; and a control channel including an -oscillator generating current at a frequency of an order of magnitude different from that of the signals, a third amplifier coupled to said oscillator and adapted to double the frequency of the` current from said oscillator, a rectifier and a direct-current amplifier; the output of said second ampli-I fier being coupled to said rectier and the output of said rectifier being coupled to said directcurrent amplifier, connections between said direct-currentfamplifier and said third amplier and connections between the output of said third amplifier and said gain control element.

2. In aA signalling system, a signal attenuator having theA characteristic that its attenuating effect increases with increase of currentthrough it, a signal-frequency amplifier coupled to the output of' said attenuator,V a rectifier coupled to the output of said signal-frequency amplifier, a source of control current at a frequency considerably higher than the frequency of the signals, a control-current amplifier for amplifying said control current, the output of the control-current amplifier being coupled to said signal attenuator, a direct-current amplifier connected to the output of said rectifier, and connections from the output of said direct-current amplifier to the anode 'circuit of said control-current amplifier, the direct-current'potential lsupplied by said direct-current amplifier being the dominant source ofanode potential for saidl control-current amplifier.

3. In va signalling system, a signal channel adapted to transmit signals at frequencies between approximately 30 and 200 cycles per second, said signal channel including a first signal amplifier,v a, signal gain-control element having the Acharacteristic that its effective resistance decreases with increase of current through it, a second signal amplifier and a signal utilization device coupled together in the order named; a control channel including an oscillator generating current at approximately 200,000 cycles per second, a third amplifier connected to amplify current from said oscillator and arranged to double the frequency thereof, a rectifier coupled to the output of said second amplien'the output of said rectier being connected to a direct current amplifier, connections between said direct current amplifier and said third amplifier, and connections between the output of said third amplifier and said gain control element, whereby the gain is effectively regulated automatically in accordance with the amplitude of the signals.

4. In a signalling system, a signal channel adapted to pass only low frequencies, a signal attenuator comprising an attenuating element` connected effectively in said channel,4 said element having the characteristic that the effective attenuation thereby varies With change of current therethrough,a source of control current at a frequency of an order of magnitude one thousand times that of the signals, control means including a frequency doubler coupling current from said source to said element, and a variable source of direct currentI connected to control the output of said frequency doubler.

5. In a signalling system, a low-frequency signal channel, a signal attenuator comprising a pair of resistance elements connected effectively in shunt with said channel, said 'elements having the characteristic that the effective resistance thereof decreases with increase of current therethrough, a source of current at a fixed frequency considerably higher than that of the signals, coupling means tuned substantially to said fixed frequency connected to couple said source and said elements in common whereby current at said higher frequency is caused to now through said elements and current at said low frequency is prevented from flowing through said elements, and control means for adjusting the magnitude of said current including a vacuum tube the grid circuit of which is connected to said source and the anode circuit of which includes said coupling means and an adjustable source of direct-current potential.

6. In a signalling system, a signal channel, a signal attenuator comprising an attenuating element connected effectively in said channel, said element having the characteristic that the effective attenuation thereby varies with change of current therethrough, a source of control current at a frequency of an order of magnitude greatly higher than that of the signals, a thermionic amplifier the input of which is connected to the source of control current and the output of which is coupled to said attenuating element, a rectifier lcoupled to said signal channel, the

output of said rectifier eiectively providing the entire source of direct-current anode potential connected to the anode of said thermionic amplier, said direct current anode potential being greatly lower than the potential specified for normal operation of said amplifier, and means for varying said anode potential and thus the attenuation of the signals.

7. In a low-frequency alternating-current transmission system, an attenuating element effective to control the transmission characteristics of said system, said element comprising a device of which the leffective resistance varies according to the current therethrough, a source of alternating current connected to said device of constant frequency and variable in magnitude, the frequency of the current of said source being greatly higher than the highest frequency of the current in said transmission system, a vacuum tube a grid of which is coupled to said source of high-frequency current and the anode of Which is coupled to said device so as to cause a highfrequency current to ow therethrough, and a rectifier coupled directly to the output of said transmission system and connected to effectively supply the entire anode potential for said vacuum tube by rectification of said low-frequency current, so as to automatically vary the magnitude of said high-frequency current in accordance with the magnitude of the low-frequency current at the output of said transmission system, and means for separating the currents of said two different frequencies whereby to prevent distortion of the low-frequency current in said transmission system.

LE ROY C. PASLAY. 

